Infection and Immunity Division, The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria 3052, Australia.
Proc Natl Acad Sci U S A. 2011 Jun 14;108(24):9869-74. doi: 10.1073/pnas.1018927108. Epub 2011 May 31.
Malaria parasite cell motility is a process that is dependent on the dynamic turnover of parasite-derived actin filaments. Despite its central role, actin's polymerization state is controlled by a set of identifiable regulators that is markedly reduced compared with those of other eukaryotic cells. In Plasmodium falciparum, the most virulent species that affects humans, this minimal repertoire includes two members of the actin-depolymerizing factor/cofilin (AC) family of proteins, P. falciparum actin-depolymerizing factor 1 (PfADF1) and P. falciparum actin-depolymerizing factor 2. This essential class of actin regulator is involved in the control of filament dynamics at multiple levels, from monomer binding through to filament depolymerization and severing. Previous biochemical analyses have suggested that PfADF1 sequesters monomeric actin but, unlike most eukaryotic counterparts, has limited potential to bind or depolymerize filaments. The molecular basis for these unusual properties and implications for parasite cell motility have not been established. Here we present the crystal structure of an apicomplexan AC protein, PfADF1. We show that PfADF1 lacks critical residues previously implicated as essential for AC-mediated actin filament binding and disassembly, having a substantially reduced filament-binding loop and C-terminal α4 helix. Despite this divergence in structure, we demonstrate that PfADF1 is capable of efficient actin filament severing. Furthermore, this severing occurs despite PfADF1's low binding affinity for filaments. Comparative structural analysis along with biochemical and microscopy evidence establishes that severing is reliant on the availability of an exposed basic residue in the filament-binding loop, a conserved minimal requirement that defines AC-mediated filament disassembly across eukaryotic cells.
疟原虫细胞的运动能力依赖于寄生虫源性肌动蛋白丝的动态转化。尽管它具有核心作用，但肌动蛋白的聚合状态是由一组可识别的调节剂控制的，与其他真核细胞相比，这些调节剂明显减少。在恶性疟原虫（影响人类的最致命物种）中，这种最小的蛋白质组包括肌动蛋白解聚因子/原肌球蛋白（AC）家族的两个成员，即疟原虫肌动蛋白解聚因子 1（PfADF1）和疟原虫肌动蛋白解聚因子 2。这种必不可少的肌动蛋白调节剂参与了从单体结合到丝的解聚和切割等多个水平的丝动力学控制。先前的生化分析表明，PfADF1 隔离了单体肌动蛋白，但与大多数真核生物的对应物不同，它结合或解聚丝的潜力有限。这些不寻常特性的分子基础及其对寄生虫细胞运动的影响尚未确定。在这里，我们展示了一种锥虫 AC 蛋白 PfADF1 的晶体结构。我们表明 PfADF1 缺乏先前被认为是 AC 介导的肌动蛋白丝结合和组装所必需的关键残基，其具有大大减少的丝结合环和 C 末端α4 螺旋。尽管结构存在这种差异，但我们证明 PfADF1 能够有效地切割肌动蛋白丝。此外，尽管 PfADF1 对丝的结合亲和力低，但仍能发生切割。比较结构分析以及生化和显微镜证据表明，切割依赖于丝结合环中暴露的碱性残基的可用性，这是一个保守的最小要求，定义了 AC 介导的整个真核细胞的丝组装。
